1. Field of the Invention
The present invention relates to a liquid droplet ejection head and an image forming apparatus, and more particularly, to a liquid droplet ejection head and an image forming apparatus in which a long line head is formed by detachably arranging a plurality of short heads in the breadthways direction of a recording medium.
2. Description of the Related Art
A liquid droplet ejection head such as an inkjet head is known in which ink is supplied to nozzles, for example, and a recording is performed by ejecting the ink in the form of minute ink droplets from the nozzles toward a recording medium. Furthermore, an inkjet printer is also known as an image forming apparatus, which forms images on recording media in accordance with print data (image data) by ejecting ink in the form of liquid droplets from a plurality of nozzles (liquid droplet ejection ports) while moving an inkjet head (liquid droplet ejection head) in which the nozzles are arranged, and the recording medium relatively to each other.
There are inkjet printers of a serial head type provided with a short head for scanning the recording medium in the breadthways direction of the recording medium to perform recording, and of a line head type provided with a long line head having a length corresponding to the maximum width of the recording medium and having nozzles arranged so as to correspond to the full dimension of this length.
In a printer using a serial head, recording is performed by recording one line (or a few lines) by scanning the recording medium with the head in the breadthways direction of the recording medium, and conveying the recording medium in a direction perpendicular to the breadthways direction through a distance corresponding to the number of lines recorded by one scan in the breadthways direction, each time one line (or a few lines) is recorded. Therefore, it not only requires a conveyance system, such as a carriage, which carries the short head in the breadthways direction of the recording medium, but also requires complicated scanning control of the movement of the carriage and the recording medium.
On the other hand, in a printer using a line head, it is possible to record an image onto the full surface of the recording medium by simply scanning the recording medium in a direction perpendicular to the direction of arrangement of the nozzles, and therefore it does not require a conveyance system, such as a carriage for carrying a short head, and neither does it require complicated scanning control of the movement of the carriage and the recording medium. Furthermore, since recording can be performed onto the whole surface of the recording medium by moving only the recording medium in a direction perpendicular to the breadthways direction thereof, it is possible to achieve a high recording speed compared to a printer that uses a serial head.
In an inkjet printer, one image is represented by combining dots formed by ink ejected from the nozzles. High image quality can be achieved by achieving high density arrangement of the nozzles provided in the inkjet head, in such a manner that the size of the dots is reduced and the number of pixels per image is increased.
In general, when manufacturing a long line head, it is essential that the nozzles are positioned to a high degree of accuracy. However, as the components becomes longer, production yield decreases due to the increase in the number of nozzles, and there is a tendency for the accumulated pitch error caused by machining to increase. In order to avoid problems of this kind, various methods have been proposed in which a long line head is manufactured by connecting together a plurality of small heads located highly accurately in position.
However, in a conventional long line head achieved by aligning short heads, since each of the short heads is completely independent, there is variation in the ink supply, unless the internal pressure is controlled in each individual short head, and hence stability declines.
For example, Japanese Patent Application Publication No. 2002-225263 discloses a long line head composed by arranging a plurality of pressure chambers in each of pressure chamber rows in an oblique direction with respect to the lengthwise direction of the head, arranging the plurality of pressure chamber rows in mutually parallel with the lengthwise direction of the head, and further arranging actuator blocks, each formed in the shape of a parallelogram having sides parallel to the lengthwise direction of the pressure chamber rows, in the lengthwise direction of the head, in such a manner that the actuator blocks are mutually separated. In this line inkjet head and a recording apparatus using same, advantages are obtained in that uniform characteristics, such as the piezoelectric characteristics and film thickness of the thin film actuators, can be achieved, fracturing of the films can be prevented, the production yield rate can be improved, the manufacturing equipment can be reduced in size, costs can be lowered, and the like. However, only the actuators are separately formed, the flow channel members must be manufactured to a large length, the accumulated pitch error becomes large, and the whole head must be replaced even if there is a fault in only one part of the flow channel.
Japanese Patent Application Publication No. 2002-36522 discloses a line head composed by forming ink flow channels so as to traverse the paper feed direction, arranging head chips alternately on either side of the ink flow channels in such a manner that ink is drawn from the ink flow channels, and connecting the head chips (short heads) together, then high-resolution printing accuracy can be achieved by means of a simple composition. However, the divided head chips are fixed to the base of a common flow channel, and therefore it is not possible to replace heads, and the whole head must be replaced if a portion of the nozzles develops an ejection failure.
Japanese Patent Application Publication No. 2002-337320 discloses a line type inkjet printer composed by alternately arranging short matrix type inkjet heads, which each form independently serviceable and replaceable units, then the total number of alternately arranged short head is counted up, the odd-numbered heads are fixed in position, and the even-numbered heads are allowed an adjustment width of 1 pixel, in such a manner that print width errors, or print irregularities at the joints between heads, can be avoided. The cross-sectional area of the common liquid chambers is increased due to the matrix arrangement, ink of high viscosity can be ejected, and the respective heads can be replaced individually; however, liquid still remains inside the head when a head is replaced and hence maintenance properties are poor.
Japanese Patent Application Publication No. 2001-96734 discloses that a long head having a substantially large number of nozzles is achieved by using a plurality of short heads, then a long head length can be achieved by detachably arranging short heads on a sub carriage, in such a manner that the regions corresponding to the joints between the short heads, where there are no nozzles, are printed by shifting the position of the heads and performing a further scanning action. However, the heads are completely independent and their internal pressures are controlled individually, and furthermore, since there are gaps without nozzles at the joints between heads, then it is not possible to print one image in a single pass. Moreover, similarly to Japanese Patent Application Publication No. 2002-337320, ink remains inside the head when a head is replaced and hence maintenance properties are poor.
Furthermore, it is important that the ink is uniformly supplied to the respective pressure chambers at high speed, when a head is formed to a large length. Japanese Patent Application Publication No. 6-143602 discloses that one end of a pressure chamber is connected to a first reservoir, the other end of the pressure chamber is connected to a second reservoir, and the other end of the first reservoir and the other end of the second reservoir are connected to a third reservoir, ink being supplied to one reservoir and being expelled from another, thereby creating a circulation of the liquid inside the pressure chambers, then uniform, high-speed ink replenishment can be achieved, and wasteful consumption of ink can be suppressed, thus reducing running costs. However, since there are a plurality of supply restrictors in the pressure chambers, pressure loss is reduced and the reflux of ink upon ink ejection becomes large, thus leading to a problem in that a large energy is required in the actuators in order to eject ink.